Biosensors utilizing the capillary method are generally used for analyzing glucose contained in blood. For example, use is made of such a biosensor 6 as shown in FIGS. 9 and 10, which is formed with a window 61 for checking that a sample liquid is supplied into a capillary 60 (See JP-A 2001-526388, for example).
The capillary 60 is defined by a substrate 62, a spacer 63 and a cover 64. On the substrate 62, a working electrode 65 and a counter electrode 66 are formed. The working electrode 65 and the counter electrode 66 are covered by an insulating film 67 so that opposite ends 65a, 65b, 66a, 66b are exposed. The end 65a of the working electrode 65 and the end 66a of the counter electrode 66 are connected to each other by a reagent portion 68.
The window 61 is provided by forming a light-transmissive region in the cover 64. The window 61 extends continuously from a sample liquid introduction port 69a toward an air vent 69b and directly above the ends 65a, 66a of the working electrode 65 and the counter electrode 66.
In the biosensor 6, the sample liquid introduced through the sample liquid introduction port 69a moves in the capillary 60 toward the air vent 69b by capillary action. The movement of the sample liquid can be checked by visual inspection through the window 61 formed in the biosensor 6.
However, when the biosensor 6 is designed to analyze a small amount of sample, the width of the capillary 60 is set small. Further, the window 61 extends continuously from the sample liquid introduction port 69a toward the air vent 69b. Therefore, in the biosensor 6, it is sometimes difficult to check which point in the capillary 60 the sample liquid has reached. Therefore, it is not always easy for the user to check, by visual inspection, that the sample liquid has reached an intended point in the capillary 60. Further, when the insulating film 67 covering the working electrode 65 and the counter electrode 66 is colored, the color of the insulating film 67 makes the color of the sample liquid indistinctive. Also in such a case, the checking by visual inspection is difficult. Moreover, since the window 61 is formed to extend directly above the ends 65a, 66a of the working electrode 65 and the counter electrode 66, the internal structure of the biosensor 6, such as the working electrode 65 and the counter electrode 66, can be seen before the use of the sensor, which deteriorates the appearance.